Revealing structural degradation in layered structure oxides cathode of lithium ion batteries via in-situ transmission electron microscopy

Transition metal oxides with layered structure have been widely used as cathode materials for lithium-ion batteries (LIBs) which have relatively high energy density, large capacity and long life. However, in the long-term electrochemical cycle, the inevitable degradation of performance of LIBs due to structural degradation in cathodes severely restricts their large-scale practical applications. Understanding the underlying mechanism of structural degradation is the most critical scientific problem. Recently, in situ transmission electron microscopy (TEM) has become a useful tool to study the structural and compositional evolutions at atomic scale in electrochemical reactions, which provided a unique and in-depth understanding of the structural degradation. In this review, we discuss the recent advances in the in situ TEM, focusing on its role in revealing the structural degradation mechanisms in the four key places: (1) the interface between the cathodes and electrolyte; (2) the cathode surface; (3) the particle interior and (4) those induced by thermal effect. The insight gained by the in-situ TEM which is still developing at its fast pace is unique and expected to provide guidance for designing better layered cathode materials.